Graduate Thesis Or Dissertation

 

The differential response of Fraxinus pennsylvanica, Liquidambar styraciflua, and Acer saccharinum to picloram as measured by CO₂ absorption and evolution Public Deposited

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  • The tolerance of woody plants to picloram (4amino-3,5,6-trichloropicolinic acid) has been shown by field applications to vary from mild to extreme. A laboratory study was done to investigate the basic causes for this range of tolerance known to exist among three woody plant species, Fraxinus pennsylvanics, March, (green ash) resistant, Liquidambar styraoiflua L., (sweet gum) intermediate, and Acer saccharinum L., (silver maple) susceptible. Seedlings of each species were grown in a nutrient solution in a controlled environment room for at least three weeks prior to treatment with picloram. Comparable response curves of all three species were determined by measuring the change in CO₂ absorption and evolution rates as a function of picloram concentration in the nutrient solution. A second experiment measured the time-dependent change in plant CO₂ absorption and evolution at 0.1 ppm picloram. Photosynthesis and respiratory rates were measured by monitoring the change in CO₂ concentration in a closed flow system with a CO₂ gas analyzer. For plants in a similar physiological state, it was postulated that picloram-induced changes in CO₂ absorption and evolution are comparable measures of special response. The results of the second experiment show a recovery in the photosynthetic rates of green ash (B) and sweet gum (I) following 6 - 12 days of steady decline. This seems to indicate a resistance mechanism resulting from a shift in the primary biochemical pathways. Such a mechanism is not evident in silver maple. The respiration rate of sweet gum is slightly stimulated by 0.1 ppm picloram while that of silver maple is depressed to 30 per cent of its original value. This is further evidence for a relative biochemical tolerance of sweet gum. Also, an experiment designed to compare leaf absorption of 10 ppm picloram between sweet gum and silver maple showed no practical difference to exist. Inspection of the concentration-response curves (CO₂ absorption) indicates silver maple to be slightly less tolerant to picloram at 0.3 and 1.0 ppm than either sweet gum or green ash. However, the characteristic "dip" in the photosynthetic rates of sweet gum and green ash, as noted in experiment two, corresponds to the period (ten days) of picloram exposure. Therefore, sweet gum and green ash are shown to be less tolerant than is actually the case. Future work utilizing this means of measuring relative plant response among species should include a determination of this higher concentrations to damp the fluctuations. The picloram uptake pattern of each species was determined by sampling the nutrient solution at times corresponding to photosynthesis and respiration measurements. (Experiment tow) The picloram concentration was determined with gas chromatography. Green ash was shwed a slow accumulation of picloram while both silver maple and sweet gum showed similar patterns of initial picloram uptake followed by a release back to the nutrient solution. The release by silver maple is thought to be due to general plant deterioration with a resulting flow along the concentration gradient. The release from sweet gum is thought to be the result of active exudation processes which may also be a resistance mechanism.
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